Hi Guys (and Gals),
A question for someone more technically minded than me.

I assume that the amount of fuel air mix drawn into a cylinder will be equal to the displacement (plus a little extra that is pulled through into the exhaust port).

If this is the case, then the same volume of fuel air mix will be drawn through, irrespective of whether you have 28s or 35s on.

Does that mean that the 28s restrict the maximum revs due to the limit of the litres of flow through the carb, due to the cross sectional diameter (compared to the 35s with a 45% greater cross sectional area).

Does this also mean that the 125cc ish of fuel air mix pulled through per cycle, still only requires the same ratio of oil/fuel irrespective of the carb size?

Think of it like this. Turn your bath tap on compared to your basin tap. The water will pour out of basin tap with more of a jet "spray" pattern. Stick your finger over the end and it sprays out even faster, same water pressure, smaller diameter pipe. Do the same with the bath tap and it's all a bit pathetic, same water pressure slower flow rate. Now stick a pump on the water and increase the pressure. The basin tap can only flow at it's max capacity governed by the bore (max flow rate) where as the bath tap can now use it's bigger bore to increase it's flow rate, and now flow more water.
A 28mm carb (basin tap) and is perfectly adequate, but when you need to flow the max amount of mixture you need a bigger bore carb (bath tap). Until you need that max flow rate i.e. WOT at max revs, the 28mm will be better, but be a limiting factor on out and out flow rate. That is way race engines need the bigger bore carbs, but the difference is marginal.
The 2nd limiting factor is how much mixture you can get into and out of the engine. Leave the plug out of your basin and your bath and tun the taps on. Water will fill up in your basin as you can't get rid of it fast enough (1 1/4" waste pipe) where as the bath never fills up (1 1/2" waste pipe)
Stick the bath tap on the basin and you can never open the tap fully as it will overflow. Stick the basin tap on the bath and it will never fill up.
So what you need is a bath tap and a bath waste on your basin
Well that's how I understand it and yes I am a plumber.

Hi Jan,
Is that because the 28s are at their maximum flow when the bike is at WOT, compared to the 35s which are not, but at lower revs the larger carb has a lower velocity for the fuel air mix?
Cheers
Bernie

don't know exactly why, and do not realy care. If I know what to use when, I'm happy.

bigger carb's and the shorter the inlet the better for high HP and visa versa (all within limits of course).
through a 35 can flow more than through a 28, so there must be some thruth to that.
smaller carbs have higher velocity which of course must also be important for something. It must have something to do for being beter at lower rev's but wouldnt know exatly why.

You know it's cold outside when you trip over dog shit instead of standing in it.

The velocity of fuel, and mixture, thru a carb is dependent on the pressure drop between atmosphere (air box) and the inside of your engine. If you make this pressure drop bigger it sucks fuel harder. 2 ways, increase the pressure outside (pressurise the air box) or lower the pressure inside the engine by making more space for fresh mixture (by which I mean get rid of the old mixture) to get in, make it easier for it to get in and make the piston move faster. Too large a crank case volume and you just cant fill it, and that alters the pressure drop.

Small carbs hit their max flow rate at a slower rate, they are therefore working at their best over a greater range of engine speeds. As with the basin taps once it's flowing at it's max it can't flow any faster, it's a 28mm bore hole

Big carbs only hit their max flow rate at a higher rate, they are only working efficiently at a very high engine speed, you can suck more air thru a 35mm hole, but you need a much bigger pressure drop to do it properly.

You can lengthen or shorten the inlet tract to help with the pressure drop

Max driveability long inlet tract small carb, flattest power curve, probably more torque over a wider rev range

Max power big carb short inlet tract, but it won't have a wide power band

But it's not just about putting big carbs on, you have to adjust everything else to let them work. You could write a book about carb tuning. Ask Eric Gorr

The R has a different oil pump to go with the 35s... That suggests a different flow rate and the impact that can have on air/fuel mix? Should the standard oil pump be a problem when your running 35s on a tuned S engine?

I'm working away or I would check my data specs for oil flow rates....

The R has a different oil pump to go with the 35s... That suggests a different flow rate and the impact that can have on air/fuel mix? Should the standard oil pump be a problem when your running 35s on a tuned S engine?

I'm working away or I would check my data specs for oil flow rates....

Hi Shaun,

I would love to know what the flow rates are on your "R" oil pump, I would expect that it was an off the shelf from Mikuni when the "Big K " were finishing off the design specs, something like the one on a Suzuki T500 or perhaps a twin 400cc bike. (I cant remember which one of the big Jap manufacturers used to make their own oil pumps).

But still, it is strange that the volume of fuel/air mix would be the same on an S as an R, I can feel another headache coming on

The R has a different oil pump to go with the 35s... That suggests a different flow rate and the impact that can have on air/fuel mix? Should the standard oil pump be a problem when your running 35s on a tuned S engine?

I'm working away or I would check my data specs for oil flow rates....

Hi Shaun,

I would love to know what the flow rates are on your "R" oil pump, I would expect that it was an off the shelf from Mikuni when the "Big K " were finishing off the design specs, something like the one on a Suzuki T500 or perhaps a twin 400cc bike. (I cant remember which one of the big Jap manufacturers used to make their own oil pumps).

But still, it is strange that the volume of fuel/air mix would be the same on an S as an R, I can feel another headache coming on

Cheers,
Bernie

Bernie less of the technical venturi principle and concentrate on more polishing. Know your position man!

Yes, that's true. I also notice the part numbers are different for KR-1 and KR-1S oil pumps.

If the KR-1R 35mm carbs flow more air and more fuel, it stands to reason that more oil is required to maintain the correct petrol:oil ratio. However, I have no idea why there should be any differences between KR-1 and KR-1S oil pumps. Perhaps the differences are not related to pump performance - anyone know?

It may be of interest to anyone fitting 35mm carbs to know whether it is possible to modify a KR-1 or KR-1S oil pump to KR-1R spec. Unfortunately I can't help because; a) I have only KR-1 oil pumps and b) I don't use oil pumps because I don't trust them - if they stop working, you don't notice until it is too late!